Apparatus for depositing epitaxial semiconductor from the liquid phase

ABSTRACT

A furnace boat has a plurality of wells therein. A first slide movably extends through the boat along and across the bottom of the wells. The first slide carries a substrate on which layers of semiconductor material are to be deposited. A second slide movable extends through the boat and across the wells adjacent the open tops of the wells. The second slide supports the starter deposition materials. A wafer extends partially across each well adjacent each slide, and an inclined guide plate extends partially across each well from adjacent the second slide to an edge of the wafer. Molten deposition material is deposited from the second slide onto the inclined guide plate and rolls into an end of the space between the wafer and the substrate carried on the first slide. The deposition material is then sucked into the space between the wafer and substrate by capillary action. Upon cooling the furnace boat, the semiconductor material will deposit from the deposition material onto the substrate. The first slide can be moved to carry the substrate into each well where a separate epitaxial layer can be deposited on the substrate.

This is a division, of application Ser. No. 879,488, filed Feb. 21,1978, now U.S. Pat. No. 4,123,302.

The present invention relates to a method and apparatus for depositingepitaxial layers of a semiconductor material on a substrate by a liquidphase deposition technique, and particularly to such a method andapparatus for depositing thin layers of the semiconductor material.

A technique which has come into use for making certain types ofsemiconductor devices, particularly semiconductor devices made of thegroup III-V semiconductor materials and their alloys, such as lightemitting devices and transferred electron devices, is known as "liquidphase epitaxy." Liquid phase epitaxy is a method for depositing anepitaxial layer of a single crystalline semiconductor material on asubstrate wherein a surface of the substrate is brought into contactwith a solution of a semiconductive material dissolved in a molten metalsolvent, the solution is cooled so that a portion of the semiconductormaterial in the solution precipitates and deposits on the substrate asan epitaxial layer, and the remainder of the solution is removed fromthe substrate. The solution may also contain a conductivity modifierwhich deposits with the semiconductor material to provide an epitaxiallayer of a desired conductivity type. Two or more epitaxial layers canbe deposited one on top of the other to form a semiconductor device of adesired construction including a semiconductor device having a PNjunction between adjacent epitaxial layers of opposite conductivitytype.

U.S. Pat. No. 3,565,702 to H. Nelson, issued Feb. 23, 1971, entitled"DEPOSITING SUCCESSIVE EPITAXIAL SEMICONDUCTIVE LAYERS FROM THE LIQUIDPHASE," describes a method and apparatus for depositing one or moreepitaxial layers by liquid phase epitaxy and is particularly useful fordepositing a plurality of epitaxial layers in succession. The apparatusincludes a furnace boat of a refractory material having a plurality ofspaced wells in its top surface and a slide of refractory materialmovable in a passage which extends across the bottom of the wells. Inthe use of this apparatus, a solution is provided in a well and asubstrate is placed in a recess in the slide. The slide is then moved tobring the substrate into the bottom of the well so that the surface ofthe substrate is brought into contact with the solution. When theepitaxial layer is deposited on the substrate, the slide is moved tocarry the substrate out of the well. To deposit a plurality of epitaxiallayers on the substrate, separate solutions are provided in separatewells and the substrate is carried by the slide to each of the wells insuccession to deposit the epitaxial layers on the substrate.

Heretofore, in depositing an epitaxial layer by the liquid phase epitaxytechnique, a large volume of the solution was used to assure goodcoverage of the entire surface on which the epitaxial layer wasdeposited. The use of a large volume of the solution creates certainundesirable effects. When the large volume of the solution is cooled,precipitation of the semiconductor material in the solution takes placethroughout the solution. The semiconductor material which precipitatesadjacent the surface of the substrate deposits thereon to form theepitaxial layer. However, the semiconductor material which precipitatesin the portion of the solution spaced from the surface of the substrateforms platelets of the semiconductor material. These platelets cancreate local instabilities in the surface morphology of the substrate,yielding an uneven surface of the epitaxial layer. Also, in thedeposition of an epitaxial layer by the liquid phase epitaxy techniques,the volume of the solution determines the thickness of the epitaxiallayer which is deposited per degree reduction of the temperature of thesolution. The greater the volume of the solution, the thicker theepitaxial layer deposited per degree reduction of the temperature. Thus,with the use of a large volume of the solution, it is difficult tocontrol the decrease of the temperature of the solution precisely enoughto deposit very thin epitaxial layers.

In order to prevent the formation of platelets so as to achieveepitaxial layers having smooth surfaces and to permit the deposition ofthin epitaxial layers, it would be desirable to use small volumes of thesolutions. However, it has been found that to merely reduce the volumeof the solution sufficiently to overcome the problems of the largevolume is not satisfactory. It has been found that the surface tensionof the materials generally used in the liquid phase epitaxy solutionscauses the small volume of the solution to ball up into a sphericalsection so that the solution does not cover the entire surface of anaverage size substrate. Thus, the small volume solution would notuniformly deposit an epitaxial layer over the entire surface of thesubstrate.

A method and apparatus for achieving small volume of depositionsolutions to achieve thin epitaxial layers is shown and described inU.S. Pat. No. 3,753,801 to H. F. Lockwood et al., issued Aug. 31, 1973,entitled "METHOD OF DEPOSITING EPITAXIAL SEMICONDUCTOR LAYERS FROM THELIQUID PHASE." In this method and apparatus a weight is placed on thedeposition solution in each well to spread out the solution across theentire area of the well and to hold the solution against the substratewhen the substrate is in the well. Although this method and apparatus ingeneral are satisfactory, there is a tendency for the solution in eachwell to be carried along with the coated substrate when the substrate ismoved into the next well because of the force applied to the solution bythe weight. Such a carrying over of the solution can cause acontamination of the solution in the next well, particularly since eachsolution is of small volume.

In the Drawings:

FIG. 1 is a cross-sectional view of a form of an apparatus of thepresent invention for carrying out the method of the present invention.

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1.

Referring to FIG. 1, a form of the apparatus of the present invention isgenerally designated as 10. The apparatus 10 comprises a refractoryfurnace boat 12 of an inert material, such as graphite, having threespaced wells 14, 16 and 18, in its upper surface. A passage 20 extendslongitudinally through the boat 12 from one end to the other and extendsacross the bottoms of the wells 14, 16 and 18. A first slide 22 of arefractory material, such as graphite, movably extends through thepassage 20 so that the top surface of the slide forms the bottom surfaceof the wells 14, 16 and 18. The slide 22 has a recess 24 in its uppersurface adjacent one end of the slide. A second slide 26 of a refractorymaterial, such as graphite, movably extends through a passage 28 whichextends longitudinally through the boat 12 and crosses each of the wells14, 16 and 18 adjacent to but spaced slightly from the open tops of thewells.

A separate wafer 30 extends partially across each of the wells 14, 16and 18 adjacent and spaced slightly from the top surface of the firstslide 22. As shown in FIG. 2, each wafer 30 is spaced slightly from onewall of its well so as to provide an opening into the space between thewafer 30 and the first slide 22. A separate inclined guide plate 32 of arefractory material, such as graphite, extends partially across eachwell 14, 16 and 18. Each guide plate 32 is inclined downwardly from justbelow the second slide 26 to the edge of the wafer 30 which is spacedfrom the aforementioned one wall of the well.

To carry out the method of the present invention, a separate charge 34,36 and 38 is placed in each of the wells 14, 16 and 18, respectively, onthe second slide 26. Each of the charges 34, 36 and 38 is a mixture of asemiconductor material of the epitaxial layer to be deposited, a solventfor the semiconductor material and, if the epitaxial layer is to be aparticular conductivity type, a conductivity modifier. For example, todeposit epitaxial layers of gallium arsenide the semiconductor materialwould be gallium arsenide, the solvent, which is generally metal, couldbe gallium and the conductivity modifier could be either tellurium ortin for an N type layer or zinc, germanium or magnesium for a P typelayer. If the semiconductor material to be deposited is gallium aluminumarsenide, aluminum would be included with the gallium aresenide. Thesemiconductor material and the conductivity modifier are present ingranulated solid form at room temperature. Since certain of the metalsolvents which can be used, such as gallium, having a meltingtemperature close to room temperature, the melting temperature ofgallium being about 30° C., the metal solvent may be present in eithergranulated solid form or in liquid form depending on the ambienttemperature where the method is being carried out. The proportions ofthe ingredients of each of the charges 34, 36 and 38 is preferably suchthat when the semiconductor material is dissolved in the molten metalsolvent, the resulting solution will be saturated with the semiconductormaterial.

Each of the wafers 30 is preferably of the same semiconductor materialas contained in the particular charge 34, 36 or 38 in the respectivewell. Each wafer 30 is spaced from the first slide 22 a distance suchthat there is only a small volume of space therebetween. By small volumeit is meant that there is only enough space to contain a thin layer ofthe deposition material. A substrate 40 of a material suitable forepitaxial deposition is placed in the recess 24 in the first slide. Therecess 24 is large enough to allow the substrate 40 to lie flat therein.

The loaded furnace boat 12 is then placed in a furnace tube (not shown)and a flow of high purity hydrogen is provided through the furnace tubeand over the furnace boat 12. The heating means for the furnace tube isturned on to heat the contents of the furnace boat 12 to a temperatureabove the melting temperature of the ingredients of the charges, forexample, between 800° C. and 950° C. for gallium aluminum aresenide andgallium arsenide. The temperature is maintained long enough to insurecomplete melting and homogenization of the ingredients of the charges34, 36 and 38.

The first slide 22 is then moved in the direction of the arrow 42 inFIG. 1 until the substrate 40 is within the first well 14. The secondslide 26 is then moved in the same direction causing the molten charge34, which is now a deposition solution, to drop onto the guide plate 32.The molten charge 34 will then roll down the guide plate 32 into the endof the space between the wafer 30 and the first slide 22. The moltencharge 34 will then be sucked into the space between the wafer 30 andthe first slide 22 by capillary action until the molten chargecompletely fills this space to provide a thin layer of depositionmaterial. Since the wafer 30 is of the semiconductor material in thecharge, which is also similar to or the same as the material of thesubstrate 40, the melting characteristics between the molten charge andeach of the wafer 30 and the substrate 40 is substantially uniform sothat the molten charge will flow easily into the space by capillaryaction. The substrate 40 is now in contact with a deposition solutionwhich is exactly saturated with the semiconductor material and which isof small volume.

The heating means for the furnace tube is then either turned off orreduced in temperature to cool the furnace boat 12 and its contents.Cooling the exactly saturated deposition solution in the first wellcauses some of the semiconductor material in the deposition solution toprecipitate and deposit on the surface of the substrate 40 to form afirst epitaxial layer. During the deposition of the semiconductormaterial some of the conductivity modifiers in the deposition solutionbecome incorporated in the lattice of the first epitaxial layer toprovide the first epitaxial layer with a desired conductivity type.Since the deposition source is in the form of a thin layer, the coolingof the deposition source results only in the deposition of theprecipitated semiconductor material on the surface of the substrate 40with only a minimum of undesirable platlets being formed in thesolution. Also, only a small amount of semiconductor material isdeposited on the substrate 40 so that a thin epitaxial layer of thesemiconductor material can be easily deposited on the substrate. At thesame time that the semiconductor material is deposited on the substrate40 some of the semiconductor material will also deposit on the wafer 30.However, this does not adversely affect the deposition on the substrate.

The first slide 22 is then again moved in the direction of the arrow 42to bring the substrate 40 with the first epitaxial layer thereon intothe second well 16. The second slide 26 is also moved in the directionof the arrow 42 to drop the molten charge 36 onto the guide plate 32 inthe well 16. The molten charge 36 will roll into the end of the spacebetween the wafer 30 and the first slide 22 and will be sucked into thisspaced by capillary action. A second epitaxial layer will then bedeposited on the substrate 40 in the manner as previously described. Athird epitaxial layer of semiconductor material can be deposited on thesubstrate 40 by moving the substrate 40 into the well 18 and droppingthe molten charge 38 from the second slide 26.

Thus there is provided a method and apparatus for depositing, by liquidphase epitaxy, a semiconductor layer from a thin layer of the depositionmaterial. This method and apparatus has the advantage that there is noforce being applied to the deposition material to press it against thesubstrate. Thus there is little tendency for the deposition material ineach well to be pulled over with the substrate into the next well sothat the chance of contaminating the deposition solution in the nextwell is reduced. Also, the space between each wafer and the first slidedetermines the volume of the deposition solution which contacts thesubstrate. Therefore the volume of the deposition solution which isbrought into contact with the substrate, which determines the amount ofsemiconductor material which is deposited, can be accurately controlledeven if the amount of material used in the charge is not accuratelycontrolled. Also, since when the substrate is moved from one well to thenext, it is moved into an empty well, the charge being dropped onto thesubstrate later, the temperature of the furnace boat can be adjusted atsuch times without affecting the substrate or the semiconductor layerthereon.

What is claimed is:
 1. Apparatus for depositing on a substrate at leastone epitaxial layer of a semiconductor material comprisinga boat of arefractory material having at least one well therein, means for moving asubstrate into and out of said well adjacent the bottom of the well,means extending partially across the well adjacent the bottom of thewell and adapted to form a space of small volume between it and thesubstrate when the substrate is in the well to provide a capillaryaction between the substrate and the said extending means, and means forreleasably supporting a deposition solution forming charge adjacent theopen top of the well.
 2. Apparatus in accordance with claim 1 in whichthe means for moving the substrate into and out of the well is a firstslide which movably extends through the furnace boat and across the wellat the bottom of the well.
 3. Apparatus in accordance with claim 2 inwhich the means for releasably supporting the charge is a second slidemovably extending through the furnace boat and across the well adjacentthe top open end of the well.
 4. Apparatus in accordance with claim 3 inwhich the means extending partially across the well is a wafer of thesemiconductor material which is in closely spaced relation to the firstslide and has an edge spaced from a wall of the well so as to provide apassage into the space between the source wafer and the first slide. 5.Apparatus in accordance with claim 4 including a guide plate extendingpartially across the well at an incline for just below the second slideto the edge of the wafer which is spaced from the wall of the well. 6.Apparatus in accordance with claim 5 in which the furnace boat has aplurality of spaced wells therein, each of the slides extends across allof the wells, a separate wafer extends partially across each well and aseparate inclined guide plate extending partially across each well.